Subtract.
Explanation: There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of tenths: $\begin{aligned} &\phantom{=}6.5 - 1\\\\ &=6.5 - 1.0\\\\ &=65\text{ tenths} - 10\text{ tenths}\\\\ &=55\text{ tenths}\\\\ &=5.5 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Ones $.$ Tenths ${6}$ $.$ $5$ ${1}$ $.$ $0$ Now, let's subtract each place value column. Ones $.$ Tenths ${6}$ $.$ $5$ $-$ $1$ $.$ $0$ $\underline{~~~~}$ $\underline{~~~~}$ $5$ $.$ $5$ The answer $5.5 = 6.5 -1$